Add Darwin+Arm64 toolchain support

This just sets up the toolchain and allows Darwin+Arm64 to be specified
as a HostCross target. These variants will not be exported to Make, or
be installed on a Soong-only build. A future CL will add support for
universal binaries using these variants.

This config is a bit stranger than the regular 64/32 multilib, as it's
two primary 64-bit configs. And on a Darwin/X86 machine, the Arm64
versions are HostCross (doesn't work on the current machines), while a
Darwin/Arm64 machine, either version works (if Rosetta is installed).

Bug: 203607969
Change-Id: Iacaed77d267773672da027cd74917e33fb1c1e94

rust/config/Android.bp

@@ -16,7 +16,7 @@ bootstrap_go_package {
         "lints.go",
         "toolchain.go",
         "allowed_list.go",
-        "x86_darwin_host.go",
+        "darwin_host.go",
         "x86_linux_bionic_host.go",
         "x86_linux_host.go",
         "x86_device.go",